Thanks for suggestion, but I tried that before I spent $700 changing the tank. The dealer assures me that they tested every line and valve before resorting to tank change. The original tank had a small dent in bottom, probably near where filler tube terminates. The tank change was the last theory the dealer had.
Dealer has now given up, but isn't giving back payment for faulty diagnosis/work.
Can anyone offer some tests to further diagnose? Thanks!